In general, this invention relates to a method and means for sealing or bonding plastics to themselves or to plastics of different material and thicknesses, and more particularly to an improved method and apparatus for sealing difficult to seal plastic films and the like.
Some of the better known methods of bonding various plastic films and the like are the use of adhesive/solvents, dielectric and thermal heating, and ultrasonic vibrations. The sealing or bonding of plastics by adhesives or solvents is achieved by spraying, brushing or dipping the areas to be bonded, then clamping them together until they are dried. Although the initial investment of this system of bonding is low, disadvantages are found in the time cycle that prevails from the application of the solvent or adhesive until it is fully dried. Furthermore, the effectiveness and permanency of the adhesive or solvent bond is often seriously impaired in the presence of a warm and humid atmosphere in which bonded films are frequently used and stored. Additionally, the solvents and adhesives are all somewhat toxic; therefore, care has to be taken to prevent operator inhalation and direct skin contact. This usually calls for elaborate ventilation and washroom facilities. Another disadvantage of the adhesive solvent system of bonding plastics is the inconsistent quality of the bond.
Dielectric heating is the principle employed in electronic heat sealing of plastic films and fabrics. In this method, radio frequency energy is induced into the work area by the presence of a dielectric field produced from a source of high frequency voltage of suitable magnitude for the thickness of the work load. The work piece must possess a loss characteristic that enables the dielectric field to force the work piece to rise in temperature. The loss characteristic, or power factor, has to be of sufficient magnitude to allow ample heating to occur without the need of excessive RF voltages. The molecular structure of the plastic material determines the presence and the extent of the necessary polarization, which is the root of the heating process. Rapid alternations in the polarity of the field cause the molecules that have polar movement to attempt to follow those alternations, and as a result of their rapid agitation, the friction loss creates the heating effect desired. Dielectric heat sealing has a number of disadvantages and limitations including the cost of the high frequency generators; the great accuracy that must be exercised to ensure that the press bed and heat are flat and planoparallel; and only materials requiring low potentials can be heated while others such as polyethylene cannot be heated at all.
The thermal heat bonding of plastics is achieved by the application of heat to selected areas on the outside surfaces of the plastic pieces. Time is then allowed for the interface to reach fusing temperatures. A large temperature differential can exist between the outside heated surface and the bonded surfaces. Distortion and embrittlement can result, and this is especially true with thick materials.
The ultrasonic method of bonding plastics uses a source of electrical energy having the desired frequency which is converted into a vibratory mechanical force, by using special materials having piezoelectric or magnetostrictive properties. Ultrasonic apparatus includes a power supply, a sonic converter and a horn. The power supply changes the 60 cycle line voltage to 20,000 cycles, and the sonic converter changes the 20,000 cycles of electrical energy into mechanical oscillations at the same frequency by means of a piezoelectric element made of lead zirconate titanate. Converter efficiencies range from 75 to 90 percent, and available mechanical output power ranges from 2200 to 8800 in -lbs. per second.
The horn is a half-wave resonant metal section designed to transmit mechanical vibrations at an amplitude of from 0.0005 to 0.005 inches from the sonic converter to the parts being joined. In bonding or sealing of plastic films and the like, the horn is coupled to the upper part to be joined so that the part vibrates against the lower part at ultrasonic rates (20,000 cps) to produce friction and heat at the interface of the two parts. The quality of the bond formed by this ultrasonic method depends on the pressure of the tool on the work surfaces, the length of time which the vibrations are impressed on the work, the amplitude of vibration of the horn, the percentage of change in thickness of the mating surfaces, and the type of plastics. Therefore, it can be seen that ultrasonic sealing/bonding does require strict control of various parameters in order to ensure quality sealing. Other detrimental aspects of ultrasonic sealing that require close scrutiny are the ill effects that may result from the operation of ultrasonic equipment such as hearing loss, and other physiological effects which include fatigue, nausea, pain and blood changes which are attributable to airborne noise radiated by the equipment, as well as any local damage resulting from direct contact with an ultrasonically vibrating device. Additionally, the ultrasonic apparatus is costly and requires skilled personnel for its operation and maintenance.
In order to overcome the foregoing problems and potential health hazards, an improved method and apparatus for bonding plastic films and the like is hereinafter described.